1. Field of the Invention
The present invention relates to foams prepared from compositions comprising polypropylene (hereinafter referred to as "PP") resin and having remarkably improved extrusion foaming characteristics and to a process for producing the same, and more particularly to foams of PP resin composition prepared by kneading at a high temperature and high pressure a composition comprising PP resin, 1,2-polybutadiene (hereinafter referred to as "1,2-PB") resin and a blowing agent, thereafter cooling the composition and extruding the cooled composition into a low pressure zone, the extruded foams being characterized in that when dissolved in hot xylene, the foams have an insoluble content of at least 2% by weight based on the whole composition and a soluble 1,2-PB resin content of up to 30% by weight based on the whole composition. The invention further relates to a process for producing same foams.
2. Description of the Prior Art
Processes are known for producing extruded foams from polyolefins or like thermoplastic resins by melting and kneading a composition of such resin and a blowing agent at a high temperature and high pressure as by an extruder, cooling the composition to a temperature suitable for foaming and extruding the composition into a low pressure zone for foaming.
Japanese Patent Publication No. 4341/1960, for example, discloses a process for preparing extruded foams from a polyolefin resin with use of sym-dichlorotetrafluoroethane as a blowing agent. More particularly said patent publication teaches a process for producing a cellular PP foam by heating highly crystalline PP resin and sym-dichlorotetrafluoroethane in a pressure-resistant steel tube container at a temperature of 175.degree. C. for 8 hours, and thereafter extruding the resulting mixture into the atmosphere in an exploding fashion with high pressure nitrogen gas introduced into the tube.
Japanese Patent Publication No. 41474/1971 discloses a process for producing foams composed of polyhedral closed cells not smaller than 500.mu. and having a density of up to 0.03 g/cm.sup.3 from a composition comprising PP resin or like crystalline organic linear polymer and a large quantity of activating liquid by flash extrusion foaming.
While these conventional processes afford foams of PP resin, the extrusion foaming operation of the former process involves low formability, produces cells of uneven structure and gives surface irregularities and poor appearance to the resulting foam, thus yielding products of reduced commercial value. With the latter process, the foams obtained are limited to the shape of filaments or a thin sheet, and the use of a large amount of activating liquid as a blowing agent inevitably gives foams expanded to an exceedingly high degree of 60 to 100 times. Thus the process involves extreme difficulties in producing under controlled conditions foams expanded to a foaming degree of 10 to 50 times and having a high degree of usefulness.
Stated more specifically, the conventional techniques for extruding and foaming highly crystalline PP resin are greatly limited in the range of temperatures for maintaining the resin at a viscoelastic strength suitable for foaming, so that it is almost impossible to maintain the resin at a properly controlled foaming temperature. Great difficulties have therefore been experienced in stably producing foams under controlled conditions having a foaming degree of 10 to 50 times with a neat appearance and uniform cellular structure.
Accordingly, it has been proposed to blend some other polymer with PP resin or to use a special PP resin as an improvement in the non-cross-linking extrusion process. Japanese Patent Publication No. 49825/1972, for example, discloses a process for efficiently preparing a foam having uniform minute cells from a mixture of polyethylene, PP resin or like polyolefin and 0.3 to 40 parts by weight of poly-cis-butadiene rubber admixed therewith. Japanese Patent Publication No. 4945/1973 further states that a highly foamed product having uniform minute cells can be prepared from a mixture of a chemical blowing agent and a special PP component containing at least 15% of n-heptane-soluble PP (which appears to be predominantly atactic) and having an intrinsic viscosity (.eta.) of 3.0 to 10.0 by foaming the mixture with an extruder operating under a back pressure of at least 200 kg/cm.sup.2 in the extruder and a temperature up to 200.degree. C. (resin temperature at the die outlet). With respect to these processes, however, Japanese Patent Publication No. 49825/1972, which includes an example wherein low-density polyethylene, amenable to a homogeneous high-degree foaming technique, is used, emphasizes the effect of foaming nuclei as achieved by the use of a very small amount of polybutadiene, and mentions the use of a small amount of water which leads to greatly stabilized foaming. Thus, the prior art, which contemplates formation of minute cells with stability, is entirely distinct from the present invention in object and concept. Additionally, poly-cis-butadiene having poor thermal stability, when admixed with PP resin, gives reduced thermal stability at a high temperature of 80.degree. to 130.degree. C., thus totally offsetting the important feature of the foamed PP resin--its high resistance to heat. The special PP resin ingredient used in the technique disclosed in Japanese Patent Publication No. 4945/1973 contains at least 15% of n-heptane soluble resin and is therefore invariably lower than usual PP resin in crystallinity and melting point. Consequently, the PP resin ingredient has considerably lower heat resistance and mechanical strength, affording foams having reduced heat resistance and mechanical strength.
In recent years there is a growing commercial demand for foams which have higher resistance to heat and lower water absorbing properties than known polyethylene or polystyrene foams and which also possess heat insulating and shock absorbing effects. Accordingly, various techniques have been developed in the art for producing highly foamed products by the non-crosslinking extrusion of high density polyethylene, polypropylene and like resins having higher crystallinity and greater heat resistance than other polyolefin resins. These techniques nevertheless still remain to be improved and have not found wide application.
In view of the present situation described above, we have: (a) conducted intensive research on techniques for producing foams having a high foaming degree of 10 to 50 times from PP resin which has especially high resistance to heat among highly crystalline polyolefins resins; and (b) developed foams, as well as a process for producing the same, which have outstanding heat resistance, a uniform cellular structure, attractive appearance and high foaming degree by overcoming the foregoing drawback of PP resin that it has a narrow range of proper extrusion foaming temperatures, without necessitating the complex steps needed for the conventional cross-linking process.